1. Field of the Invention
The invention relates to a method and apparatus for measuring the height of at least one density layer in the material bed of a density sorting machine, particularly a jigging machine for processing mineral or solid mixtures.
2. The Prior Art
Density grading machines, such as jigging machines, are utilized to sort solid mixtures based on relative densities of material. Typically, a jigging machine receives a supply of solid material in a jig bed which is mechanically vibrated or jiggled in order to displace heavier particles towards lower levels of the material or jig bed.
In a jigging machine for sorting wet mineral mixtures, particularly raw clean coal, German OS No. 2734736 describes means for measuring the relative height of the layer of heavy materials in a jig bed using a flotation element. The heavy materials are discharged from the jig bed responsive to float height enabling a throughflow of heavy materials. The float is a hollow body of a large size compared to the particles in the jig bed and carries a transmission rod extending vertically upward from the upper surface of the float. The transmission rod maintains the float on a vertical path and its height allows an observer to determine the layer height of heavy materials in the jig bed. In this arrangement, however, the float behaves only approximately like the jigged material itself. Changes in material density along the height of the jig bed are not immediately and precisely registered by the float, which can result in faulty measurements and ill-timed discharges.
Furthermore, this float arrangement is not well-suited to measure the boundary layer between layers of coal which exhibit only slight differences in density among the particles. Without the necessary precision, misplaced materials could arise in situations having a separate discharge of the coal layers. A hollow body float construction is also subject to wear, such that its overall density may change thus producing inaccurate layer height readings from the jig bed.
The present invention is drawn to an apparatus and method which overcomes these disadvantages with a simple device for determining precisely where layers of only slight density differences are separated.